Ineed help on yoshi's island on the DS?

Answer 1

Bonus levels:
After completing the game, an extra level for each world will appear.

Bandit Mini-Games
Go to the stage select screen, hold select, then press L,L,B,A,R. You will be taken to the Bandit Mini-Games menu screen with all six games where you can get unlimited power-ups.

Finish all eight levels in each world with a 100% completion to unlock a ninth level in each world.

Bonus games:
Hold Select and press L(2), B, A, R at the level selection screen. A list of bonus games will appear. If you win the selected game you will get a prize.

Lucky 777:
Get your total points for a world to equal to exactly 777, to get 7 extra lives.

Special Yoshi eggs:
To get special Yoshi eggs with out having to find special boxes, fire a Yoshi egg in an area that will cause the egg to bounce off the walls and back to you. When the egg returns to you, quickly use Yoshis tongue at the egg. If successful, you should have a special Yoshi egg.

Level 1-7: Extra lives:
Pick up a Rollie-Pollie Guy with your tounge. Do not turn him into an egg or spit him out for anyone until you get to the pipe that spits out Shy-Guys. Once there, spit the Rollie-Pollie Guy out into a 1 (square) hole on the right of the pipe. Once placed, jump onto the platform above and to the right of it. Let the Shy-Guys jump out of the pipe and onto the Rollie-Pollie Guy. Points will accumulate and will eventually give you 1-Ups continually. After a few hours, you will have 999 lives.

Answer 2

Yoshi's Island DS Unlockables:
New characters
You can get four new babies to ride on yoshi. They are baby peach, baby DK, baby Wario, and baby Bowser. Peach is unlocked at world 1-5, Dk at 2-1, Wario at3-5, and Bowser at 4-5. (note- Baby Wario and Baby Bowser will not remain with you forever, but peach and dk will.
Unlockable --> How to Unlock
Baby Peach --> World 1-5
Baby DK --> World 2-1
Baby Wario --> World 3-5
Baby Bowser --> World 4-5

Time Trial
Once you've beaten the game, you will be rewarded the Time Trial mode. To access Time Trial enter the File Menu like you would starting up a game. Your completed game file will have a star icon and will have the word "Clear!" under where the world should be. Click on your file and you will notice "Time Trial" on the bottom right corner where a question mark used to be. In Time Trial mode, you will be rewarded 5 extra levels, one for each world. You will also be able to play those levels with a black Yoshi.
Unlockable --> How to Unlock
Time Trial Mode --> Complete the game once
Black Yoshi --> Play the secret levels in Time Trial

Unlock Extra levels
To unlock the extra levels you will need to collect a 100 points on 8 stages (Except secret levels) of that world. Meaning you must get 20 red coins, 5 flowers and 30 stars to get 100 points. All together 800 points. Unlockable --> How to Unlock
Extra 1 --> Get 800 points for world 1
Extra 2 --> Get 800 points for world 2
Extra 3 --> Get 800 points for world 3
Extra 4 --> Get 800 points for world 4
Extra 5 --> Get 800 points for world 5

Character Coins Bonus
In the game there is a coin with one of the baby's faces on it in each level and if you manage to find all of the coins in each world you unlock the harder version of a minigame in the minigames menu!
Unlockable --> How to Unlock
Tulip Shooter (Hard) --> Get all character coins in World 1
Flutter Challenge (Hard) --> Get all character coins in World 2
Bouncy Maze (Hard) --> Get all character coins in World 3
Speed Eating (Hard) --> Get all character coins in World 4
Egg Toss (Hard) --> Get all character coins in World 5

Secret Bonus Challenge Panels
At the end of each level there is a ring that can have 0 to 5 flowers on it depending on how many flowers you found in the level. When you jump in this ring there will be an imaginary ball that circles around the ring and if it stopped on a flower you would get warped to a bonus stage. There is another way to get one of these 5 bonus levels however and here's how...
Unlockable --> How to Unlock
Flip Cards --> Get a total of 700 points in World 1
Scratch and Match --> Get a total of 700 points in World 2
Match Cards --> Get a total of 700 points in World 3
Roulette --> Get a total of 700 points in World 4
Slot Machine --> Get a total of 700 points in World 5